A new method for the preparation of MgAl layered double hydroxide-copper metal–organic frameworks structures: application to electrocatalytic oxidation of formaldehyde

In this research, we present a novel design protocol for the in-situ synthesis of MgAl layered double hydroxide-copper metal–organic frameworks (LDH-MOFs) nanocomposite based on the electrocoagulation process and chemical method. The overall goal in this project is the primary synthesis of para-phthalic acid (PTA) intercalated MgAl-LDH with Cu (II) ions to produce the paddle-wheel like Cu-(PTA) MOFs nanocrystals on/in the MgAl-LDH structure. The physicochemical properties of final product; Cu-(PTA) MOFs/MgAl-LDH, were characterized by the surface analysis and chemical identification methods (SEM, EDX, TEM, XRD, BET, FTIR, CHN, DLS, etc.). The Cu-(PTA) MOFs/MgAl-LDH nanocomposite was used to modification of the carbon paste electrode (CPE); Cu-(PTA) MOFs/MgAl-LDH/CPE. The electrochemical performance of Cu-(PTA) MOFs/MgAl-LDH/CPE was demonstrated through the utilization of electrochemical methods. The results show a stable redox behavior of the Cu (III)/Cu (II) at the surface of Cu-(PTA) MOFs/MgAl-LDH/CPE in alkaline medium (aqueous 0.1 M NaOH electrolyte). Then, the Cu-(PTA) MOFs/MgAl-LDH/CPE was used as a new electrocatalyst toward the oxidation of formaldehyde (FA). Electrochemical data show that the Cu-(PTA) MOFs/MgAl-LDH/CPE exhibits superior electrocatalytic performance on the oxidation of FA. Also the diffusion coefficient, exchange current density (J°) and mean value of catalytic rate constant (Kcat) were found to be 1.18 × 10–6 cm2 s−1, 23 mA cm-2 and 0.4537 × 104 cm3 mol−1 s−1, respectively. In general, it can be said the Cu-(PTA) MOFs/MgAl-LDHs is promising candidate for applications in direct formaldehyde fuel cells.


Materials and instruments
The chemicals were used in this research including: Sodium chloride (NaCl) (99%), para-phthalic acid (PTA), copper (II) nitrate trihydrate [Cu(NO 3 ) 2 .3H 2 O], ethanol, dimethylformamide, formaldehyde (CH 2 O), sodium hydroxide (NaOH) purchased from commercial sources and used without further purification.The aluminum and magnesium plates were purchased from a reputable company.The solutions were made with double distilled water.The surface morphology and structural of the prepared materials was evaluated by using a scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) (MIRA3 FEG-Tescan) and transmission electron microscopy (TEM) (Carl Ziess AG-Zeiss EM900).The X-ray diffraction (XRD) patterns were collected within the range of 5.3-80° 2θ on a Bruker D 8 advance diffractometer with Cu K radiation (λ = 0.154056 nm).Thermogravimetric analysis (TGA) was performed using SETARAM SETSYS 16/18 thermal analyzer heating instruments (heating rate of 5 °C/min) in the nitrogen flow atmosphere (25 mL per min).Specific surface area and pore size distribution of the samples was determined by analyzing N 2 physisorption isotherms (BELSORP Mini II).For determination of percentage C, N and H, the CHN analysis was carried out by Euro EA elemental

Procedure for the preparation of unmodified and modified CPE
In this work the preparation of unmodified CPE is as the same as which described in our previous work 56 .The mixing of the graphite powder and liquid paraffin was done by means of a pestle and a mortar in order to obtain a homogeneous paste, which was used to fill the working electrode hole.Hole filling is made in small portions when each of them being pressed intimately before adding the next one.Then the CPE was smoothed onto a white, clean and soft paper in order to remove the excess of carbon paste.The electrical contact was made via copper wire inserted into the syringe and into the back of the composite past.CPE lefts unused for a certain time (15 h) to allow their final homogenization to proceed.This process of "self-homogenization" has been confirmed experimentally; freshly homogenized CPEs often exhibit a rather unstable behavior.For the preparation of the modified CPE, the Cu-(PTA) MOFs/MgAl-LDH was grounded with graphite powder with the ratio 3:70 (w/w) for achieving a uniformly wetted paste and then the paste was packed into working electrode hole (ca.3.4 mm i.d. and 10 cm long) and pressed thoroughly by mechanical force.Then, the excess paste, if present, remove carefully and smoothing the surface on a weighing paper 57 .The schematic preparation steps of the Cu-(PTA) MOFs/MgAl-LDH/CPE was presented in Fig. 1.As above, modified CPEs left unused for 15 h to allow their final homogenization to proceed and dried.

Physicochemical characterization of synthesized materials
The surface morphology, elemental composition, internal and crystal structure of the synthesized materials were analyzed by a variety of diagnostic tools to obtain the morphological, structural and elemental information about the resulted nanocomposite.Typical morphology of the synthesized MgAl-(PTA) LDH and Cu-(PTA) MOFs/ MgAl-LDH was depicted by SEM and shown in Fig. 2A,B.The captured image A exhibits aggregated and a flakelike morphology.Figure 2B shows the SEM image of the Cu-(PTA) MOFs/MgAl-LDH with a distribution of uniforms microcrystals of Cu-(PTA) MOFs/MgAl-LDH composition.It seems that hydroxide groups existence interlayer of LDH, regulated formation of Cu-(PTA) MOFs particles through mechanism-oriented growth on MgAl-(PTA) LDH surface 58,59 .Furthermore, the spectra related to elemental analysis (EDX) of MgAl-(PTA) LDH and Cu-(PTA) MOFs/MgAl-LDH are also shown in Fig. 2C,D 3A,B.It is quite clear that, the translucent plate-like morphology could be clearly in MgAl-(PTA) LDH (Fig. 3A) with a size about 30-50 nm arising from insertion of PTA particles between LDH layers and the change in the structure of the LDH plates.In the second stage, by introducing copper into the interlayer structure of LDH and creating a new structure (Cu-(PTA) MOFs), it can be observed (Fig. 3B) that a series of dense structures were created with a deformed hexagonal structure and the most of the particles are normal in size (20-50 nm).The particle size distribution of the MgAl-(PTA) LDH and Cu-(PTA) MOFs/MgAl-LDH in respective aqueous suspensions was analyzed by dynamic light scattering (DLS) technique and presented in Fig. 3C.The particle sized results of the MgAl-(PTA) LDH (red continuous line), with size ranged between 113 and 400 nm and mean particle size about of 175 nm.In Fig. 3C, green dashed line shows the DLS result of the Cu-(PTA) MOFs/MgAl-LDH with particle size which was determined between 70 and 150 nm, and mean particle size of 105 nm.The decrease in particle size of Cu-(PTA) MOFs/MgAl-LDH vs MgAl-(PTA) LDH due to the entry of copper ions and formation of MOFs structure is in good agreement with the results of the TEM analysis.In the following, the fundamental physicochemical properties (specific surface area and pore-size distribution) of the MgAl-(PTA) LDH and Cu-(PTA) MOFs/MgAl-LDH were studied from N 2 adsorption-desorption plots and the Barret-Joyner-Halenda (BJH) curve with physisorption isotherms as shown in Fig. 4 A and B, respectively.The samples displayed a type IV isotherm with H3 hysteresis loops and a type IV isotherm with sharp uptakes and H 3 -type hysteresis loops.In terms of experimental research, the most important data obtained from the analysis of Brunauer-Emmett-Teller (BET) curve: specific surface area and the average pore size distribution.In this case, the BET specific surface area of the MgAl-(PTA) LDH was found to be 137 m 2 /g, while for the Cu-(PTA) MOFs/MgAl-LDH, it was 111 m 2 /g.Moreover, the average pore size distribution of the MgAl-(PTA) LDH was determined by BHJ plot as 18 nm, while pore size of the Cu-(PTA) MOFs/MgAl-LDH was obtained as ≈10 nm that the sample was composed of hierarchical porous material.Therefore, the formation of the Cu-(PTA) MOFs composite within MgAl-(PTA) LDH leads to the reduction of the cavity space in the interlayer of LDH, which results in a decrease in the surface area and volume of cavity of the final compounds.In addition, the reduction of the average diameter of the cavity from 18 nm for MgAl-(PTA) LDH to 10 nm for Cu-(PTA) MOFs/MgAl-LDH is evidence of the functionalization of the holes of the MgAl-(PTA) LDH 60,61 .In order to examine the crystal structure of the synthesized materials, the XRD technique was employed and results shown in Fig. 4C.The patterns exhibited five peaks at 2θ of 12.36, 24.13, 35.2, 46, 53.23, 62.12 and 62.96 corresponding to (113), (110), (018), (015), (009), (006) and (003) planes of MgAl-(PTA) LDH (pattern a) 44 .In addition, XRD pattern of the Cu-(PTA) MOFs/MgAl-LDH (pattern b) in Fig. 4C contains six main peaks in the 2θ of 8.44°, 10.36°, 15.56°, 16.76°, 26.72°and 30.56° corresponding to planes of (001), (003), (010), (002), ( 006) and (009), respectively 45 .Therefore, in general, it can be said that the  62,63 .Also, to study and identify the functional groups in MgAl-(PTA) LDH and Cu-(PTA) MOFs/MgAl-LDH, the Fourier transform infrared spectroscopy (FT-IR) technique was used.According to the obtained results in Fig. 5A, the peaks in the wavelength number of 673 and 786 cm -1 are related to the stretching vibrations of Al-O and Mg-O functional groups 64,65 .Furthermore, the broad and strong peak at the wavelength number 3453 cm -1 is related to the O-H bond, which indicates the presence of water molecules in the between layers in brucite layers of MgAl-(PTA) LDH 66 .Also, the peaks at 1368, 1570, 1351 and 673 cm -1 are attributed to the symmetric and asymmetric stretching states of the carboxylate group (OCO -), stretching vibrations of the carboxylate group (OCO -) and C-H vibrational modes in MgAl-(PTA) LDH 67 .In addition, the peaks in 1416 and 1569 cm -1 in Fig. 5B are attributed to the coordination of PTA ligand to Cu (II) ion in Cu-(PTA) MOFs/MgAl-LDH composite 68 .The two sharp and strong peaks observed at the 868 and 1398 cm -1 in the resulting spectrum of Cu-(PTA) MOFs/MgAl-LDH are related to the stretching vibrations of the interlayer nitrate anions and confirm the fact that the nitrate group in the LDH interlayer plates with bonds MOFs have been replaced.Also, the peak observed at the wavelength number of 755 cm -1 is related to the lattice vibrations of M-O and M-O-M bonds in the octahedral planes of LDH-MOF bonds in the synthetic Cu-(PTA) MOFs/MgAl-LDH which is close to similar reported papers 47,68 .The CHN analyses of MgAl-(PTA)LDH and Cu-(PTA) MOFs/MgAl-LDH nanocomposite are given in Table 1.Based on the results, the carbon content for the Cu-(PTA) MOFs/MgAl-LDH has increased from 31.713 to 37.668% after the Cu-MOFs formation and intercalated into the interlamellar gallery.Additionally, the content of nitrogen in the MgAl-(PTA) LDH after the intercalation process has significantly increasing from the 2.461 to 7.502%, implying that the nitrate ions have been exchanged with linker onions in the MgAl-(PTA) LDH due to reaction PTA with Cu cations introduce on the interlamellar gallery of LDH compound 69 .The appearance of two sharp and strong peaks in the resulting spectrum of Cu-(PTA) MOFs/MgAl-LDH in FT-IR (Fig. 5B) is related to the interlayer nitrate anions confirm this phenomenon and is in good agreement with the results of the CHN analysis results.

Electrochemical behavior of the Cu-(PTA) MOFs/MgAl-LDH in alkaline media
Initially, for the activation of the Cu-(PTA) MOFs/MgAl-LDH/CPE, its cyclic voltammograms (CVs) (five cycles) were recorded in appropriate range of potential from 0.0 to 1200 mV vs. Ag/AgCl in 0.1 M NaOH at a scan rate of 50 mV s -1 and results shown in Fig. 6A.The CVs show that the appearance of the voltammograms changes during the number of cycles for the modified electrode.As can be seen, in the first scan, an anodic peak appears in the area of formation of active copper species.In the next cycles, the current of anodic peak is reduced, so that after five cycles, it reaches almost to zero current with an irreversible behavior.Therefore, it can be said that, during successive cycling, the oxidation reaction starts with the interaction between the copper (II) ions present in the Cu-(PTA) MOFs/MgAl-LDH/CPE and the sodium ions in the solution according to the following reaction 57 : As stated by Eq. (1), the electroactive ions, Cu (II) ions at the modified electrode surface are oxidized to Cu (III) species.In the next cycles, due to the entry of OH -ions into the MOFs/MgAl-LDH composite structure and the conversion of Cu (II) to Cu (OH) 2 and Cu (OH) 2 to Cu (III), were take place at the surface of modified electrode according to the following reaction 71,72 : Figure 6B illustrates the typical CVs of the Cu-(PTA) MOFs/MgAl-LDH/CPE at various scan rates (10-200 mV s -1 ) in 0.1 M NaOH solution.The results show that the anodic peak currents are proportional with the scan rate.The obtained curve, inset a, in Fig. 6B confirms the liner dependence of I p versus ν 1/2 , which indicates a diffusion-controlled process 73 .Also, the transfer coefficient was calculated from the plot of peak potentials (E p ) vs logarithm of scan rate (Fig. 6B inset b) and Eq. ( 4) as α = 0.19.
(1) The surface coverage (Г*) (normalized to the geometric area) of the active spices at the Cu-(PTA) MOFs/ MgAl-LDH/CPE was measured from the slope of anodic peak current (I p ) vs scan rate as the 7.72 × 10 -5 mol cm -2 for n = 1: In Eq. 5, A is the working electrode area, ν is the scan rate and Γ* is the surface coverage of the active species 74 .To obtain the electroactive surface area of the Cu-(PTA) MOFs/MgAl-LDH/CPE, the [Fe(CN) 6 ] −3/−4 ions (5 mM) was used as a probe redox system.According to the Randles-Sevcik equation (Eq.6): where: (I p ) is the peak current, (n) is the number of electrons transferred in the reaction, (D) is the diffusion coefficient and C is the concentration of [Fe(CN) 6 ] −3/−4 , (v) is the scan rate (V/s) and A is the active surface area of the modified electrode (cm 2 ).The CVs of the Cu-(PTA) MOFs/MgAl-LDH/CPE were recorded in different scan rate (not shown here) and the results were analyzed; I p vs ν 1/2 .From the slope, the active surface areas of the Cu-(PTA) MOFs/MgAl-LDH/CPE was determined as 0.46 cm 2 .
(5)   In fact, it can be suggested that the bonded copper ions in the MgAl-(PTA)LDH structure act as intermediate ions in the FA electrooxidation reaction 77 .On the other hand, the redox Cu couple Cu(II)/Cu(III) has play as a mediator role on heterogeneous catalytic oxidation of FA.Indeed, FA is firstly oxidized to format ions in alkaline medium, and then the format ions are oxidized to carbon dioxide, Eq. ( 8) 77 .www.nature.com/scientificreports/Furthermore, to investigate the relationship between the concentration of the background electrolyte solution and the anodic peak current of FA electrooxidation, the CVs of the Cu-(PTA) MOFs/MgAl-LDH were drawn in the presence of 66 mM FA and different concentrations of NaOH and the obtained results shown in Fig. 7B.It was observed that, (inset of Fig. 7B), with increasing the NaOH concentration from 0.025 to 0.1 M the anodic peak currents increase, but in the higher concentration of NaOH (> 0.1), the observed anodic peak current rapidly decreased.It seems that hydroxide ions compete with FA in occupying the sites of Cu-(PTA) MOFs/MgAl-LDH composite composition and reduces the active sites 75,78 .Therefore, the concentration of 0.1 M NaOH was chosen as the optimal concentration.Also, the CVs of the Cu-(PTA) MOFs/MgAl-LDH/CPE were investigated in 0.1 M NaOH solution containing different concentrations of FA in the potential range − 0.2 to 1.2 V (all CVs at scan rate of 50 mV s -1 ) and the obtained results shown in Fig. 8. Inset of Fig. 8 shows the oxidation peak current of FA vs the concentration of FA.As can be seen, the anodic peak current increases with increasing FA concentrations with linear correlation (R 2 = 0.9922).
By referring to reference electrochemistry books 79 , it can be said that the cyclic voltammetry technique could provide information about the charge transfer processes, electrode stability, and effect of chemical reactions on the electrode reactions.To determine the nature of the anodic peak current dependency on the electrocatalytic oxidation of FA, the CVs of the Cu-(PTA) MOFs/MgAl-LDH were recorded in the presence of 66 mM FA + 0.1 M NaOH in different scan rates (10-250 mVs) and the results shown in Fig. 9.As can be seen from the CVs, by increasing the scan rate of potential, the potential of the anodic peak current (E p ) of FA shifts to more positive potentials, which indicates the existence of a kinetic limitation in the electrode reaction process between Cu-(PTA) MOFs/MgAl-LDH and FA 80 .Also, according to the information obtained from the data processing, drawn graphs and the plot of I p vs. square root of scan rate (υ 1/2 ) (Fig. 9 inset a), the anodic peak currents show linear dependency with the square root of scan rate.This behavior is the characteristic of a diffusion-controlled process (i.e. the spontaneous transfer of the electroactive species from regions of higher concentrations to regions of lower concentrations near surface of the electrode) 81 .The second CV segment is the E p versus log υ (Fig. 9 inset b) and slope of dEp/dlog υ which was found to be 87.201,so b = 177.402.From the Eq. ( 9) and assuming one electron transfer in rate-determining step, n α = 1, a charge transfer coefficient (α) of the FA oxidation was calculated as 0.61 82 : Also, from the Tafel plot (Fig. 9 inset c) at a low scan rate of 5 mVs −1 , the value of the exchange current density (J°) was obtained as 23 mA cm -2 .
Furthermore, chronoamperometry (CA) was used to measure the diffusion coefficient and catalytic rate constant of electrooxidation reaction FA at the Cu-(PTA) MOFs/MgAl-LDH and the obtained results shown  www.nature.com/scientificreports/ in Fig. 10.Also, the chronoamperograms curves for various FA concentrations were recorded at the Cu-(PTA) MOFs/MgAl-LDH (Fig. 10 curve 1 in the absence of the FA and curves 2-8 in presence of FA at the concentration ranges of 26-106 mM, respectively).Value of diffusion coefficient was calculated from Cottrell equation and from the results in inset (a) of Fig. 10.The value of diffusion coefficient was found as 1.18 × 10 -6 cm 2 s -1 .On the other hand, the catalytic rate constant (k cat ), for the electrooxidation reaction of FA at Cu-(PTA) MOFs/ MgAl-LDH was obtained according to the Galus method and Eq. ( 10) 75,82,83 : where I cat and I L are the currents at the modified electrode in the presence and absence of FA, respectively, and γ = kC o t [C o is the bulk concentration of FA (mol cm -3 )], k cat catalytic rate constant (cm 3 mol -1 s -1 ) and t is the time elapsed (s).Based on the plot of the slopes of the straight lines against the FA concentration (inset b of Fig. 10), dependence of I cat /I L to the t 0.5 (inset c of Fig. 10) and Eq. ( 10), the average value of k cat is obtained as 0.4537 × 10 4 cm 3 mol −1 s −184-86 .
The steady-state polarization curves for the electrooxidation of FA on the Cu-(PTA) MOFs/MgAl-LDH at different concentrations of FA in 0.1 M NaOH solution were recorded and presented in Fig. 11A.During the tests, to avoid the interference of mass transfer in the kinetics measurements rotation speed of the electrode was fixed at 3000 r/min.It can be seen that; the oxidation process begins around potential 404 mV (vs.Ag/AgCl) and reached a highest level at potential at 818 mV (vs.Ag/AgCl) while oxygen evolution begins at higher potential values.In the course of reaction, the coverage of Cu III increases and reaches a stable state level [Eq.( 11)] 71,72 .
In this case, the oxidation current based on the Eq. ( 12) can be calculated according to the following Eq.( 13).
Figure 11B demonstrate the plots of reverse of i against reverse of C FA which obtained through the curve with a straight line at different potentials and Eq. ( 15) [85][86][87][88] .
(10) I cat I L = γ 0.5 π 0.5 = (K cat Cπ) 0.5 t 0.5 (11) It is noteworthy that both the slopes and intercepts in Fig. 11B are both dependent on the value of the potential.The slope of the graph was plotted against exp(-nFE/RT) with n = 1 and presented in Fig. 11C.By referring to this diagram and Eqs.(13, 14 and 15) the rate constant of reaction, k 1 Γ and ratio of k 0 -1 /k 0 1 were calculated as 2.866 × 10 -9 cm.s -1 and 1.97 × 10 -7 , respectively.In the following, the variation of the intercepts of the lines in Fig. 11C vs applied potential in a semi-log scale is shown in Fig. 11D.Using this graph and Eqs.(13, 14 and 15)  the magnitude of k 0 1 was obtained as 2.47 × 10 -9 mol cm -2 s -1 .

Stability study of the Cu-(PTA) MOFs/MgAl-LDH/CPE
The stability of the Cu-(PTA) MOFs/MgAl-LDH/CPE after a working applied course interval of 15 days (longterm operation period in various electrochemical methods) was investigated by recording the current response of FA oxidation in the same condition.The outcomes show that, negligible changes (< 5%), among the retained current response compared with the initial current as illustrated in Fig. 12. Actually, the changes in the electrocatalytic activity are negligible and the level of current remains nearly constant (CV day1≈ CV day15 ).Considering that this material (Cu-(PTA) MOFs/MgAl-LDH) was able to show approximately constant electrocatalytic activity toward the electrooxidation of FA after fifteen days and performing various electrochemical tests during these days at the worked pH (Fig. 12), can indicates that this material (nanocomposite) has sufficient stability in these conditions.In other words, its structure has not changed during these fifteen days in the worked pH solution and various electrochemical tests.Otherwise, its electrocatalytic activity should be reduced or lost.These results demonstrated that the Cu-(PTA) MOFs/MgAl-LDH/CPE has a promising potential as a stable and efficient electrocatalyst for oxidation of FA under the optimized experimental condition.Finally, the electrochemical performance of the Cu-(PTA) MOFs/MgAl-LDH/CPE as an efficient electrocatalyst for FA electrooxidation was compared with other reported electrocatalysts and shown in Table 2.The data in Table 2 shows that the electrocatalytic performance of the Cu-(PTA) MOFs/MgAl-LDH/CPE is superior or comparable with most reported electrocatalysts toward FA electrooxidation reactions. ( . In Fig. 2C, the EDX spectrum of MgAl-(PTA) LDH shows the strong peaks of C, O, Mg and Al with weight percentages of 17.70, 54.38, 18.27 and 9.66%, respectively.Figure 2D refers to EDX spectrum of the Cu-(PTA) MOFs/MgAl-LDH which represents the growth of copper-MOFs on/in MgAl-(PTA) LDH with strong peaks of C, O, Mg, Al and Cu with weight percentages of 39.32, 34.82, 4.04, 8.16 and 13.66%, respectively.For examination of the topography of internal structure, TEM images of the MgAl-(PTA) LDH and Cu-(PTA) MOFs/MgAl-LDH were prepared and shown in Fig.
www.nature.com/scientificreports/Electrocatalytic activity of the Cu-(PTA) MOFs/MgAl-LDH/CPE toward FA oxidation In following, to investigate the electrocatalytic activity of the Cu-(PTA) MOFs/MgAl-LDH/CPE toward the oxidation of FA, the CVs of unmodified carbon paste (UCPE), MgAl-(PTA) LDH/CPE and Cu-(PTA) MOFs/ MgAl-LDH were recorded in 0.1 M NaOH solution as background electrolyte in the absence and presence of 66 mM FA and shown in Fig. 7A.As can be seen, there are no obvious redox peaks (anodic and cathodic peaks) on the voltammograms of the UCPE in 0.1 M NaOH solution without FA (curve a) and also after the addition of FA (curve b).Therefore, the UCPE is electrochemically inactive for FA oxidation.Figure 7A curve c shows the electrochemical behavior of MgAl-(PTA) LDH/CPE in the background electrolyte and curve d shows the same electrode in the presence of FA.Indeed, by modifying the CPE with MgAl-(PTA) LDH, the obtained electrode does not have a favorable electrocatalytic behavior toward the oxidation of FA.On the other hand, by inserting of the copper ions to the MgAl-(PTA)LDH/CPE and construction of the Cu-(PTA) MOFs/MgAl-LDH/CPE, the amount of anodic peak current is increased in the presence of FA (curve f) (curve e shows the CV of the same electrode in the background electrolyte) which indicates that the incorporation of Cu (II) into the MgAl-(PTA) LDH and preparation of the Cu-(PTA) MOFs/MgAl-LDH/CPE leading the oxidation of Cu (II) to Cu (III) and electrocatalysis of the FA oxidation and appearance of a high oxidation peak current, which represents the electrocatalytic behavior of the Cu-(PTA) MOFs/MgAl-LDH/CPE toward the oxidation of FA 75,76 :

2 Figure 7 .
Figure 7. (A) Cyclic voltammograms of CPE (a and b curves), MgAl-(PTA) LDH/CPE (c and d curves), Cu-(PTA) MOFs/MgAl-LDH/CPE (e and f curves) in 0.1 M NaOH solution in the absence and presence of 66 mM FA at the scan rate of 50 mV s −1 , respectively.(B) Cyclic voltammograms of the Cu-(PTA) MOFs/MgAl-LDH/CPE in different concentrations of NaOH in the presence of 66 mM FA with a scan rate of 50 mV s-1 .Inset: Anodic peak currents vs NaOH concentrations.

Figure 9 .
Figure 9. Cyclic voltammograms of the Cu-(PTA) MOFs/MgAl-LDH/CPE in 0.1 M NaOH containing 66 mM of FA at different scan rate (10-250 mV s -1 ).Inset (a): Plot of anodic peak currents with square root of scan rate, υ.Inset (b) the plot of E p vs. Log υ.Inset (c) the Tafel plot for FA oxidation at the same electrode from the CV at a scan rate of 5 mV s -1 .

Figure 12 .
Figure 12.Stability test of the Cu-(PTA) MOFs/MgAl-LDH/CPE in 0.1 M NaOH + 0.12 M FA solution in the first day (day 1) and after long-term operation period (15 days) in various electrochemical methods (day 15).

Table 2 .
Comparison of the electron-transfer coefficient (α), diffusion coefficient (D FA ), exchange current density (J°) and catalytic rate constant (k cal ) of different modified electrodes used in electrocatalytic oxidation of FA. a CPE, Carbon Paste Electrode.b GCE, Glassy Carbon Electrode.